U.S. Pat. No. 5309235, issued May 3, 1994, to Saiprasad V. Naimpally, and entitled "System and Method for Transmitting Digital Data in the Overscan Portion of a Video Signal", discloses a method of encoding quadrature amplitude modulation (QAM) signals in the overscan portions of the horizontal line of a video signal. Furthermore, the patent describes a technique to overcome the problem of ringing by doubling the symbols at each of the edges of the signal encoded in the video signal. At the receiver the added symbols are removed. This requires some extensive circuits at the receiver to discard the added symbols. It also does not overcome the problem of time shifts caused by various components in the system.
The standard technique in the industry to solve the problem of timing and synchronization is to provide a clocking sequence at the start of the line followed by a framing byte. Again, utilizing this technique requires additional circuits at the receiver to recover the clocking pulses and to synchronize to the framing byte. Further, there needs to be a way of transiting from a digital signal to an analogue signal and not having transient noise problems at the boundaries. Such problems are obviated by techniques in accordance with the principles of this invention. The doubling of symbols at the edges of the signal works when all the apparatus used have high fidelity and precise timing and the video waveforms are reproduced without distortion. Otherwise, the recovered signal will have noise causing numerous errors when the data encoded in the signal is recovered. The decoder also needs complicated circuit to discard the repeat symbols. The repeat symbols at the edges of the signal do not solve the time shifts and limited bandwidth of the receiving apparatus problems. The above problems end up in the system as noise. The circuitry at the decoder is also complicated due to the need to discard the repeat symbols.